New pneumatic - would like some advice on build and GGDT

[Mercwear]

Hello,
I have been reading the site for a while and am in the process of building my 2nd pneumatic cannon. I have been using GGDT to run some tests and want to make sure I am using the application correctly. The image below gives a brief overview of my build and a screenshot with what (I think) is a correct GGDT input and output.



My primary concern is the size of my chamber, I think it’s too small but when I use GGDT to increase my chamber volume it only gives me a slight performance bump. I am going for distance when I shoot and my ammo will be potatos.

Any guidance you can give me on the build is appreciated.
Thanks

[MrCrowley]

Sprinkler valve data should be (according to those at Spudtech):

Cv: 6 (you have it on efficiency, you need to change this back to 'Cv')
Seat Diameter: 1.1"
Piston Diameter: 1.8"
Piston Mass: 0.8oz
Vent diameter: 0.15" (0.2" if your blowgun is modified for extra flow).

Also, your projectile should also be 2" in diameter (same as your barrel), not sure why you only have it as 1.56". Even if you're using a sabot, put the diameter as 2".

[Mercwear]

Thanks for the reply MrCrowley - what are your thoughts on the size of the pressure chamber?

[Gun Freak]

I say don't use steel on the tank. Use 1.5" pvc, it can take 150 psi.

[Mercwear]

I say don't use steel on the tank. Use 1.5" pvc, it can take 150 psi.

I like the safety of steel.. I will shoot this thing off around several people (some are kids) and I just cant compromise on safety. My first pnumatic was pvc and held up to 150 just fine but there is always that off chance that it could blow up (I had it wrapped in steel mesh and duct tape but that would only slow down the shards of PVC, not stop them).

I would love to hear your thoughts on chamber size given what I am going to use this gun for (distance of potato). If I need to increase the chamber size to 2' pipe (1 foot) I can do that.

Thanks for the reply gun freak!

[Gun Freak]

Well if your primary ammo is potatoes, don't use a 2" barrel. I think that is too big. Try a 1.5" barrel about 3 feet long ( or more, but I suggested shorter because you already have a short barrel in the design) with a spud cutter on the end. For the chamber I would suggest two feet of 2" PVC or similar (metal is you want). Just play around with the numbers in GGDT and see what the smallest chamber you can use with a good fps is.

[Technician1002]

2 inch is OK of you use what they call "Bakers" in the US. They are large. They are the spuds used for baked spuds and french fries. This is why most fries are longer than your average spud.

They work great in a 2 inch barrel, are larger with more mass so they fly farther.

Regular spuds from the supermarket are too small for a 2 inch barrel.

In my location with lots of apples, I shoot many more apples than spuds. I use the free ones that have fallen off the trees.

[Mercwear]

Sprinkler valve data should be (according to those at Spudtech):

Cv: 6 (you have it on efficiency, you need to change this back to 'Cv')
Seat Diameter: 1.1"
Piston Diameter: 1.8"
Piston Mass: 0.8oz
Vent diameter: 0.15" (0.2" if your blowgun is modified for extra flow).

MrCowley,

Where did you get this info? I looked at spudtech but could not find any specifics on my valve.

Thanks

[Ragnarok]

Where did you get this info? I looked at spudtech but could not find any specifics on my valve.

The old Spudtech forum. The current Spudtech site, no longer having a forum, is unlikely to include any of that kind of data.

[Technician1002]

Some other common sprinkler valves are listed on the GGDT site. They list the specs for a 1 inch Orbit watermaster which is close to the rainbird.

http://thehalls-in-bfe.com/GGDT/

[Mercwear]

Thanks again for the replies.. I am going to stick with a 1.5 x 12in steel chamber for now and just up the PSI to around 150 to get 200+fps out of the gun =)